Polycarbonate resins are used widely as engineering plastics because of their high impact strength, thermal stability and transparency, superior electrical properties and dimensional stability, and self-extinguishing properties. However, since polycarbonate resins are molded at relatively high temperatures, they turn yellow or decrease in molecular weight by heat decomposition during the molding, resulting in a reduction in mechanical strength. This defect tends to become especially outstanding during molding at high temperatures of more than 300.degree. C. which are utilized in producing thin fabricated articles which will meet the recent need for saving resources. In particular, in the production of colored polycarbonate resins containing pigments and/or dyes, the degree of their discoloration increases, and their molecular weight decreases. Accordingly, products of the intended color or strength cannot be obtained constantly.
Organic phosphites such as triaryl phosphites or trialkyl phosphites have previously been suggested as stabilizers for preventing the discoloration or molecular weight decrease of polycarbonate resins during molding at high temperatures. Polycarbonate resins containing such phosphites have the defect that when exposed to high temperatures in air or in humidity, they become discolored or cloudy and get brittle. In an attempt to remedy this defect, the use of complex ion-type organic boron compounds (see Japanese Laid-Open Patent Publication No. 90361/78) or phosphoric acid and its esters (see U.S. Pat. No. 3,404,122) was suggested. However, these compounds do not produce a sufficient effect of preventing discoloration during molding at high temperatures.
It is an object of this invention therefore to provide a polycarbonate resin composition, especially a colored polycarbonate resin composition, which does not discolor or decrease in molecular weight during molding at high temperatures, and does not get degraded when exposed to elevated temperature in dry or humid air.
The present inventors have made various investigations in order to achieve this object, and found that a composition prepared by blending a polycarbonate resin with a benzenephosphonate not known heretofore as a heat stabilizer is free from discoloration during molding at high temperatures.